1. FIELD OF THE INVENTION
This invention relates to a process for the manufacture of dicarboxylic acid anhydrides by the oxidation of hydrocarbons. More particularly, it is directed to a process suitable for producing maleic anhydride from saturated hydrocarbons in higher yields than heretofore possible.
Maleic anhydride is of significant commerical interest throughout the world. It is used alone or in combination with other acids in the manufacture of alkyd and polyester resins. It is also a versatile intermediate for chemical synthesis. Significant quantities of maleic anhydride are produced each year to satisfy these needs.
The prior art discloses a number of processes used in the conversion of organic feedstocks to maleic anhydride. As an example, U.S. Pat. No. 3,156,705 discloses the conversion of olefins to maleic anhydride using a phosphorus-vanadium-oxygen catalyst having a phosphorus to vanadium ratio varying from 1:1 to 2:1 wherein the phosphorus is stabilized with 0.05 to 5 weight percent titanium tetrachloride to prevent phosphorus loss from the catalysts during the conversion of the olefins to maleic anhydride. U.S. Pat. No. 3,156,706 discloses a process for the conversion of olefins to maleic anhydride using a vanadium-phosphorus-oxygen catalyst which contains from 0.05 to 5 weight percent of a mixture of an alkali metal and titanium to stabilize the phosphorus in the catalyst during the conversion. Both of these patents disclose that the vanadium in the catalyst was reduced to an average valence in the range of 2.5 to 4.6 using an acid such as hydrochloric acid or oxalic acid during the preparatory steps.
A process of oxidizing saturated aliphatic hydrocarbons to maleic anhydride under controlled temperature conditions in the presence of phosphorus-vanadium-oxygen catalysts was disclosed in U.S. Pat. No. 3,293,268. One method taught in that patent for preparing catalysts comprised reacting phosphoric acid with a vanadium compound in aqueous hydrochloric acid solution, recovering the remaining solids by evaporating the solution to dryness, and then heating the solids to 300.degree. to 500.degree. C. The resulting catalysts were ground to pass a 20 mesh screen and pelletted to form tablets. The tablets were then charged to a fixed catalyst bed in a test reactor at room temperature and the reactor heated for 16 hours. Thereafter, a 0.5 volume percent butane-in-air mixture was passed through the catalyst in a fixed tube reactor at temperatures above 400.degree. C. to form maleic anhydride.
Despite the teachings in these and other references in the prior art, these teachings fail in one or more ways to achieve the results and advantages of the present invention. Although yields in excess of 20 mole percent were reported in U.S. Pat. No. 3,293,268, these yields were achieved only at temperatures between 500.degree. and 600.degree. C. and when using low butane concentrations in air. At temperatures below about 500.degree. C., the yields of maleic anhydride were reported to be less than about 12 mole percent. On the other hand, the process of the present invention can convert butane to maleic anhydride in significant yields at temperatures as low as 350.degree. C. using much stronger butane-in-air concentrations. The yields at lower operating temperatures that are achieved using the present process show that the present process is far superior to the prior art processes.
Although many workers in the prior art have disclosed processes using phosphorus-vanadium-oxygen catalysts deposited on an inert diluent, such as titania, for use in a fluidized bed or fixed tube reactor, improved yields or lower operating temperatures were not reported due to the presence of these elements, since they were usually present only as diluents and did not increase the activity of the catalysts. In stark contrast, however, to the prior art processes, the process of this invention is characterized by the conversion of saturated hydrocarbons to maleic anhydride at lower temperatures than heretofore possible.